I'm about to head out on the JMT in Sept - where I'm expecting nights to drop as low as 15 degrees. I don't have much experience with dealing with batteries and cold temps - but I know temps (both high and low) can be bad for battery life.

I'm bringing a GoPro and a camera - should I bother to remove the batteries and put them somewhere at night (or during the day if the temps drop) where they'll be insulated? Does anyone else do this when the temps or cold - or am I being paranoid?

15F ain't cold (says the Alaskan). Alkalines are okay if you're a little careful with them (in an inner jacket pocket, for instance). Fully cooled to 15F and their output is much less. UV water treatment is the most demanding thing we do and for that, you're talking body orifices to get things up to adequate output of amps.

But lithiums are lighter, higher energy, and good to -20F, so why not? Win, win, win (except they're a few times the cost). I get mine on line off eBay for 1/4 the retail price which solves that cost problem.

Emergency gear in our cars is all on lithiums. Because if it doesn't work well below 0F, it isn't emergency gear.

For the JMT, I would take lithium primary batteries. Alternatively, I would take alkalines. I do not think that I would take NiMH, which would be much more practical if I had frequent places for recharging or if I had to carry a solar recharger. NiMH does not pack quite all of the power of other battery types, and it is not very peppy in cold weather, even if it is better than nothing.

One strategy would be to take two battery sets. Use one in the camera and carry one for spare. Keep the spare in some warm pocket as much as possible. After the camera has been getting cold overnight, swap the batteries so that the warm set is now used, and put the cold set into the warm pocket.

> But lithiums are lighter, higher energy, and good to -20F, so why not?> Win, win, win (except they're a few times the cost). Actually, while the Energiser AA and AAA lithium batteries cost more per cell, they work out cheaper than alkalines over time. Yes, cheaper! They contain a lot more ampere-hours than the alkalines.

Other types of lithium batteries may not handle the cold quite as well, but they are a lot lighter.

Roger, I agree, but it may be difficult to get all of the energy out of a lithium primary battery unless you have a device that can use it properly. For example, many good headlamps are regulated. That means that they can draw power from a worn battery and keep going a lot longer. Other devices will use a battery well when it is new, but it will all suffer badly as soon as the battery voltage drops a bit.

I carry my spare batteries in an inner pocket when it's cold, if the device itself will fit in my jacket or a cargo pocket I'll do that as well. And I sleep with my devices (phone, ebook) inside my sleeping bag to help preserve the batteries.

Cold temperatures slow the chemical reaction that the battery uses to generate power. Cold temperatures generally do not cause permanent damage to the batteries.

cold nights followed by above freezing temperatures during the day will not have a noticeable impact on the life of a battery. At night when it is cold the device may show a low battery warning because of the slow chemical reaction or at worst case will not work at all. However once the temperatures warm up the battery and device will work normally.

High temperatures can cause permanent mechanical damage to the inside of a batteries and reduce the cycle life of rechargeable batteries. However in september you will not see temperatures hot enough to have any noticable effect. Even in persistent 100 degree days in the desert you probably wouldn't notice any hot temperature effect.

The only issue you have to worry about is condensation. Electricity and water don't mix. Water in a device will cause shorts that could kill the device. Also when electricity flows from on wire to another through water the wire will corrode at an accelerated rate. If a device gets wet remove the batteries immediately and don't reinstall them until you are sure the device is dry.

"Not using (or storing) batteries at 15° F won't hurt, as long as you warm up the batteries before you use them. For example, Energizer AA Alkaline batteries are OK from 0°F to 130°F."

Thats interesting - and exactly the tip I needed. I had always assumed that them being cold in non-use situations would affect the charge - but I've been wrong! I can just pull out the batteries and warm them up next to my skin before I use them, or just keep them in my pocket..

In accordance with my general gear geekiness, 10 years ago I did a rather extensive test of the performance of AA and AAA Duracell alkaline, AA and AAA Energizer NiMH rechargables (AA-2100 mAh; AAA 800 mAh), and Energizer lithium AA and AAA cells in ~20 different models of Princeton Tec flashlights and headlamps. I measured the run times of each type of cell at temperatures of 70*F, 32*F, and 0*F, and I compared the performances of Krypton, halogen and Xenon bulbs. I did minimal testing of LED bulbs.

The Xenon bulbs provided the greatest lumen output, and of course drew the most power (therefore the shortest run times until the batteries died), followed by the halogen bulbs, and then the lowly (and now rare) krypton bulbs.

Using a representative flashlight with a Xenon bulb (the Rage model, which used 4 AAA cells), the following table lists the run times of the various batteries at the different temperatures:

So it appeared to me that NiMH rechargable cells perform significantly better than alkaline at temperatures below around 40*F, whereas lithiums are not affected much at all. One significant issue relates to the self-discharge rates of the various cells, that is, how much capacity (power) they lose when sitting on the shelf over time.

I contacted both Duracell and Energizer and they provided me with this info:

Duracell alkaline cells retain 95% of their full charge after one year, and they continue to self-discharge at the rate of 5%/year. Alkalines should be stored at a temperature of 68*F. Their operating temperature range is 32*F-150*F, with a big capacity loss below 30*F (which will be partially reversed when brought back up in temperature). The output begins to slowly fade after reaching the battery's half-life.

Lithiums experience minimal self-discharge over a period of 5-10 years, and they may be stored in tenperatures ranging from 0*F-140*F. Their operating temperature range is between -40F and +140*F. The output remains steady right to the end.

Rechargable cells undergo dramatic self-discharge rates. After being fully charged, they will lose 30% of their capacity in the first 24 hours, then 1% per day after that. At temperatures above 70*F, a fully charged rechargable cell can lose up to 80% of its capacity in that first 24 hours. For long term storage, it is recommended that you fully charge these cells, then "almost" fully discarge them, then store them in a freezer bag in the refrigerator at 40*F. Rechargables also tend to fade after reaching the half-life, but they are somewhat more effective at temperatures below 32*F than alkalines.

It's possible that battery technology has changed in these past 10 years, especially with the rechargable cells. But I thought I'd offer what I learned. For all my backcountry uses, I now use Energizer lithium cells exclusively. The weight and dependability outweigh any other factors, although I cringe at the cost.